This invention relates to a fuel cell and more particularly to a fuel cell which fits miniaturization.
The fuel cells have been recently attracting keen attention as independent power generating devices on account of their high efficiency. These fuel cells are broadly classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and alkaline fuel cells which share the requirement for consuming a gaseous fuel and methanol fuel cells and hydrazine fuel cells which share the requirement for consuming a liquid fuel. Since these fuel cells are intended for power sources chiefly used for driving power generators and large equipment, they necessitate such devices as a compressor and a pump for the introduction of the gaseous or liquid fuel and an oxidizing gas into the cell proper, entail complication of system, and consume electric power for the introduction of these substances.
The fuel cells will be specifically described below with reference to the methanol fuel cell which uses methanol as a liquid fuel, for example. In the system of the methanol fuel cell, the fuel is transferred from a methanol tank by a pump to the cell proper and air as the oxidizing agent is supplied from the ambient air by a blower to the cell proper. Particularly since this cell forwards as a dissolved fuel a mixed liquid of methanol destined to serve as a fuel and an electrolyte such as, for example dilute sulfuric acid in a compressed state via a methanol controller and an acid controller to the cell proper by means of a pump, the system is complicated all the more. The other fuel cells similarly suffer from this complication. The systems of all fuel cells invariably require to use a blower and a pump for the transfer of a fuel and an oxidizing gas. This complication of system arises from the fact that the current fuel cells are intended as power sources for power generation apparatus and large equipment and consequently destined to handle large volumes of power. To fulfill the functions, they require transfer of large volumes of the fuel and the oxidizing gas and consequently necessitate such transfer devices as a pump and a blower.
As a social trend, various facilities such as OA (Office Automation) equipment, audio equipment, and radio equipment have undergone gradual miniaturization with the advance of semiconductor technologies and are required to be adapted for portability. As power sources for satisfying this requirement, handy primary batteries and secondary batteries are now in use. The primary batteries and secondary batteries, however, are functionally limited in service life. The OA and other equipment using these batteries naturally offer limited service life. In the case of the OA equipment which uses these batteries, after the batteries are exhausted, the operation of the equipment can be continued by replacing the spent batteries with new supplies. Since the primary batteries have a short service life for their weight, they are unfit for portable equipment. The secondary batteries are rechargeable after they have completed discharge. Since the recharging requires a power source for exclusive use therefor, it not only consumes much time but also compels the secondary batteries to find only limited places for service. Particularly, the OA equipment incorporating a secondary battery therein inevitably imposes a limit on the length of continued service life because the replacement of batteries which is required after completion of discharge is difficult to make. The desire to give various small battery-operated devices an increase in service life cannot be easily satisfied by mere extensions of the conventional primary and secondary batteries. In the circumstances, the desirability of developing batteries having a longer service life has been finding growing recognition.
As one measure for the solution of the problem in question, the fuel cells described above are available. The fuel cells are advantageous in that they are capable of generating electric power simply by the supply of a fuel and an oxidizing agent and further in that they allow protracted and continued generation of electric power by the supply of fuel. When they are amply miniaturized, therefore, they can serve as highly advantageous systems for the operation of such small devices as the OA equipment which call for only a small power consumption.
Since the fuel cells allow use of air as the oxidizing agent, they place no limit on the place of use or on the duration of use from the standpoint of the oxidizing agent. The use of a gas as the fuel is not appropriate for the miniaturization of a fuel cell because the amount of the gas to be required for the generation of electric power is large on account of the density thereof, notwithstanding the 0A equipment has a small power consumption. A liquid fuel has a high density as compared with a gaseous fuel and, therefore, is overwhelmingly advantageous as the fuel for the fuel cell to be used in a miniaturized OA device. When the fuel cell using a liquid fuel can be miniaturized, therefore, a power source for a miniaturized device which is capable of operating for a heretofore unattainable long duration can be realized. The problem in the realization of the power source for such a miniaturized device as mentioned above consists in the fact that since the system using the liquid fuel as conventionally practiced necessitates use of a pump for feeding the liquid fuel and a blower for feeding an oxidizing gas respectively to the cell proper as described above, the system itself is complicated and in the existing construction cannot be easily miniaturized.
Further, in the conventional fuel cells such as phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide cells, the gases which are supplied by a pump and a blower to the cell proper are introduced into an oxidizing electrode and a fuel electrode via gas channels which adjoin the electrodes mentioned above. In this case, the gas paths of the gas channels are formed each in the shape of a large groove enough to preclude the occurrence of pressure loss to the fullest possible extent from the standpoint of flowing the large volumes of fuel gas and oxidizing gas without exerting an undue load on the pump and the blower. In the gas channels of the carbonate fuel cell, for example, grooves of a depth exceeding 2 mm are generally formed. This rule similarly applies to such fuel cells as methanol fuel cells which use a liquid fuel. Particularly methanol fuel cells unlike phosphoric acid fuel cells mentioned above suffer more seriously from pressure loss than when a gas is used as the fuel because a mixture of sulfuric acid serving as the electrolyte and methanol destined to serve as the fuel is supplied to the electrolyte layer and the fuel electrode. Since the conventional fuel cells use a pump and a blower for the supply of a gas and a liquid as described above, the grooves in the gas channels are inevitably formed in large dimensions. When the grooves are formed in small dimensions, the pump and the blower are required to have large capacities. Thus, the conventional fuel cells have a problem in successfully attaining miniaturization by extension of the conventional techniques.
As a fuel cell enabled to cope with the problems mentioned above and adapted for miniaturization, the liquid fuel cell which utilizes the capillary attraction for the supply of a liquid fuel has been proposed (refer to Japanese Unexamined Patent Publication No. 66,066/1984). This liquid fuel cell is a parallel flow type cell which supplies a liquid fuel by aspiration from a fuel storing chamber disposed in a lower part upwardly in one direction to an anode by the capillary attraction of a capillary material using as a matric material an organic or inorganic fibrous material as paper, cotton, asbestos, or glass or a synthetic fibrous material as acryl or nylon and also supplies an oxidizing gas in the same direction as the fuel. This fuel cell requires the fuel storing chamber to be provided in the lower part and a gas inlet to be formed in the lower part of the cell proper to permit the supply of the oxidizing gas in the vertical direction. Thus, it is constructed so as to interpose a gap between the fuel storing chamber and the bottom surface of the stack. In this system, the capillary material is formed of fibers of appropriate flexibility so that part of the capillary material may be mechanically constricted to allow control of the supply of the fuel there-through. Further, this fuel cell is so constructed that the capillary material formed of electrically insulating substance as mentioned above may be buried in part of the collector on the anode side and held in tight contact with the collector.
The liquid fuel cell described above fits miniaturization more than the conventional fuel cells because it relies on the capillary action for the supply of the liquid fuel to the fuel electrode. It nevertheless entails a plurality of problems and, therefore, requires improvements capable of eliminating these problems. As mentioned in the relevant specification, this fuel cell has a constructional limitation that the liquid fuel is capable of permeating or penetrating the anode (fuel electrode) in the horizontal direction but is incapable of succumbing to the capillary action in the upward direction. Further, since the fuel cell of this system is so constructed as to necessitate interposition of a gap between the bottom surface of the stack and the fuel storing chamber and insertion of the fibrous capillary material in the fuel storing chamber as described above, the capillary material and the fuel storing chamber allow no easy sealing and, at the same time, the stack and the fuel storing chamber must be so constructed as to be integrally fixed. Moreover, since the fuel storing chamber must be provided in the upper part thereof with a plurality of inlets for the introduction of the capillary material in preparation for the integration, it entails the disadvantage that the construction thereof is complicated and extremely difficult to manufacture. It may be safely added that the construction for this integral fixation, the necessary for opening slits at least in the oxidizing electrode part of the bottom surface of the stack for the purpose of ensuring the flow of the oxidizing agent also adds to the complication of the construction.
Since the liquid fuel is supplied by the capillary action which is manifested in one direction from the lower part to the upper part as described above, the travel of the fuel to the upper part of the fuel electrode consumes much time and the shape of the fuel cell places a limit on the supply of the fuel by the capillary action. Generally for the purpose of increasing an electric current to be generated, such electromotive components as electrodes and electrolytic plates require an increase in area. In cases where the cell height is limited as in the present system, the electromotive components must be inevitably enlarged in width and the cell proper consequently restricted in shape. Further, this fuel cell has the capillary material of insulating substance buried in part of the collector on the fuel electrode side, the electrons which are obtained in consequence of the cell reaction inevitably flow through the collector, with the result that the electricity is concentrated and the route for the flow of the electricity is elongated possibly to the extent of inducing an electric loss.
As respects the work of clamping the fuel cell, the conventional fuel cells of large output and large area have required use of a large clamping device which is capable of exerting uniform force on the cell proper for the purpose of ensuring ample contact between the adjacent component parts of the fuel cell and enhancing the performance of the finished fuel cell. The phosphoric acid fuel cells and the molten carbonate fuel cells of the commercial grades have cell areas approximately in the range of from 5,000 to 10,000 cm.sup.2. For the purpose of clamping these fuel cells to ensure thorough contact between the adjacent component parts thereof, the clamping device to be used for the work of clamping is required to be capable of exerting a clamping load approximately in the range of from 15 to 30 tons. The clamping devices heretofore adopted for handling the conventional fuel cells, therefore, are invariably complicated and overly voluminous. Thus, they are unfit for handling fuel cells which are directed toward miniaturization.
As one major cause for complicating the conventional fuel cells, the problem in the removal of the water which occurs as a reaction product on the surface of electrode may be cited. Generally, in the fuel cell, water occurs on one of the opposed electrodes as the product of cell reaction. This water must be removed from the surface of the electrode. The water thus stagnating as the product of cell reaction on the surface of the electrode obstructs the supply of a substance for replenishment to the electrode and, as a result, impairs the efficiency of the reaction in question.
Particularly the solid polymer electrolyte fuel cell which uses as an electrolyte a protonic conductive membrane such as of perfluorocarbon sulfonic acid (product of Du Pont marketed under trademark designation of "Nafion") operates at a relatively low temperature (room temperature to 100.degree. C.) and, therefore, can be expected to serve effectively as a power source for miniaturized devices. In the fuel cell which operates at the low temperature not exceeding 100.degree. C., the problem of stagnation of the water on the surface of the electrode gains all the more in seriousness because the water produced on the oxidizing electrode occurs in a liquid state and is not easily vaporized.
In the conventional fuel cell, the recovery of the formed water has been effected by providing an air supply duct on the lateral surface and an air discharge duct on the opposite lateral surface respectively of the cell proper and causing the produced water to form dew on the wall surface of the air discharge duct. FIG. 42 schematically illustrates the conventional fuel cell.
To be specific, as illustrated in FIG. 42, an air supply duct 83 incorporating therein a blower 82 is disposed on one lateral surface and an air discharge duct 84 is disposed on the other lateral surface respectively of a fuel cell proper 81 and a formed water recovery duct 85 is disposed below the air discharge duct 84 and these ducts are installed in a cell case 88 provided with openings for an air inlet 86 and an air outlet 87. The air is supplied by the blower 82 and the air containing the formed water is transferred into the air discharge duct 84. The formed water is allowed to form dew on the inner wall surface of the air discharge duct 84 and the dew produced by the formed water is recovered in the formed water recovery tank 85 and the discharged air is forwarded through the air outlet 87 and released into the ambient air.
When the method for recovering the formed water in the manner described above is applied to such a miniaturized fuel cell as expected, the advantage derived from the high efficiency and the compactness of size is lost because the electric power for driving the blower itself and the volume of the blower are too large to be ignored. Other methods have been proposed for recovery of the formed water. The method which effects the recovery of the formed water after this water has been gasified, by nature, has poor energy efficiency because it requires supply of energy equivalent to the heat of vaporization.
To realize a miniaturized fuel cell, therefore, it becomes necessary to use a mechanism which is capable of removing the formed water from the oxidizing electrode without using extra electric power or energy.
As another cause for complicating the system of a fuel cell, a problem of construction may be cited.
FIG. 43 illustrates a common construction of superposed layers in one example of the phosphoric acid fuel cell. In this case, conducting plates 39 which are called a separator or an interconnector are interposed one each between a plurality of electromotive parts composed of an oxidizing electrode 38, an electrolyte layer 36, and a fuel electrode 37 to connect the electromotive parts in series and secure necessary voltage. This construction is not limited to the phosphoric acid fuel cell but applied effectively to the molten carbonate fuel cell and further to the solid polymer electrolyte fuel cell. In the fuel cell of this construction, the fuel or the air as an oxidizing agent which has been supplied to the cell by the use of a pump or a blower is introduced into the relevant electrodes via separators which adjoin the oxidizing electrode and the fuel electrode. In this case, from the viewpoint of flowing large volumes of such reactant substances as fuel and oxidizing gas without exerting an undue load on the pump or the blower, the paths for gas flow in the separators or the electrode plates are formed in the shape of a groove so deep as to avoid inducing pressure loss to the fullest possible extent. In the case of a fuel cell which uses a liquid fuel such as methanol, since this fuel cell flows the fuel in a liquid state unlike the phosphoric acid fuel cell, the pressure loss gains all the more in seriousness than when the fuel is used in a gaseous state.
The fuel cell of the conventional construction requires to have a considerable thickness because the paths for flowing the reactant substances must be formed as in the separators. Thus, the volume to be occupied by other than the inherent electromotive parts, i.e. the electrolyte layer, the reaction catalyst for the fuel and the oxidizing electrode, and the collectors, is suffered to increase inevitably. If the electrode plates and separators which function as paths for flow are reduced in thickness, since the narrow paths for flow still play the part of supplying the reactant substances, the pump and the blower are compelled to bear an undue load. As an inevitable consequence, these mechanisms must be enlarged.
As a measure to attain miniaturization of the fuel cell as a whole by decreasing to the fullest possible extent the volume which is occupied by other than the inherent electromotive parts, i.e. the electrolyte layer, the reaction catalyst for the fuel and the oxidizing electrode, and the collectors, a method which comprises arranging a plurality of electromotive parts vertically in the direction of thickness, namely parallelly in the lateral direction and interconnecting the terminal parts thereof in series may be conceived. The fuel cell embodying this method allows the supply of the reactant substances to and the recovery of the reaction products from the plurality of electromotive parts to be attained by the use of one empty space, obviates the necessity for separators, and does not require the electrode plates to function as paths for flow. Thus, the paths may be omitted or reduced in thickness. The concepts of this principle have been already disclosed in Japanese Unexamined Patent Publications No. 141,266/1988 and No. 141,270/1988, for example.
The fuel cells of such construction as described above, however, entail the following problems.
In the ordinary fuel cells, the reactant substances which are supplied to the electrolyte layer for the purpose of replenishing the components of electrolyte which have been lost by exudation or vaporization or preventing the electrolyte layer from drying are made to incorporate therein the same electrolyte as contained in the electrolyte layer and steam.
Particularly, the solid polymer electrolyte fuel cell which uses as an electrolyte a protonic conductive membrane such as of the aforementioned perfluorocarbon sulfonic acid (product of Du Pont marketed under trademark designation of "Nafion") among other electrolytes suffers from gradual shortage of water on the fuel electrode side and gradual decline of efficiency because water flows in conjunction with ions from the fuel electrode to the oxidizing electrode during the operation of the fuel cell. The fuel to be supplied to the fuel electrode, therefore, is replenished with a liquid electrolyte when the fuel is a liquid substance such as methanol or with steam when the fuel is hydrogen gas.
On the other electrode, formation of water takes place. In the aforementioned solid polymer fuel cell, the oxidizing electrode side has excessive water because of the presence of the water flowing from the fuel electrode in addition to the water which is formed by the electrode reaction.
The water, electrolyte which have been supplied as entrained by the reactant substances, and formed water form the cause for ionically linking or short-circuiting a plurality of electromotive elements and consequently inducing a decline in cell voltage. This problem gains in conspicuousness particularly when the water and electrolyte mentioned above happen to occur in a liquid state.
In the fuel cell of the construction having a plurality of electromotive elements arranged parallelly in the lateral direction and the terminal parts thereof interconnected in series, the problem of such voltage loss gains all the more in seriousness because the electromotive elements share a space for the supply of fuel and a space for the recovery of the product and further because the distances between the adjacent electrodes are small.
FIG. 28 is a schematic diagram illustrating two electromotive parts arranged parallelly in the lateral direction and interconnected in series. In FIG. 28, 37 stands for a fuel electrode, 60 for an electrolyte layer, and 38 for an oxidizing electrode and two electromotive parts 55 are electrically interconnected in series with a lead 57. The fuel electrodes 37a, 37b and the oxidizing electrodes 38a, 38b of the two electromotive parts adjoin each other. The supply of the fuel and the discharge of the product can be carried out in one empty space in either of the two electromotive parts. In the construction of FIG. 28, the supply of the fuel is effected through a fuel path 58 and the supply of the oxidizing gas through an oxidizing agent flow path 59.
In this case, a potential gradient is produced between the fuel electrode 37a of one of the electromotive part and the fuel electrode 37b which is equivalent in potential to the oxidizing electrode 38a of the same electromotive part. A potential gradient is similarly formed between the oxidizing electrode 38a and the oxidizing electrode 38b.
In this case, when a substance which functions as an electrolyte happens to exist on the surfaces of the two electromotive parts, i.e. the fuel flow path 58 and the oxidizing agent flow path 59, migration of ions occurs in accordance with the potential gradient mentioned above. This migration of ions functions as a leak current and induces loss of voltage.
When the miniaturization of a fuel cell is to be attained with the construction in which the plurality of electromotive parts are arranged parallelly in the lateral direction and interconnected in series and are made to share an empty space for the supply of the fuel and an empty space for the recovery of the product as described above, the voltage loss which occurs between the electromotive parts is suffered to degrade the efficiency of the fuel cell. Thus, the miniaturized fuel cell requires to decrease the voltage loss.
The fuels which are effectively used for the common fuel cells include gaseous fuels such as hydrogen gas and liquid fuels such as methanol and hydrazine, for example. The use of a gaseous fuel in the fuel cell for an OA device is unfit for the purpose of miniaturizing the fuel cell in spite of the small power consumption by the 0A device because the amount of the gaseous fuel required for power generation is very large on account of the density of the gas. The liquid fuel has a high density as compared with the gaseous fuel and constitutes itself an overwhelmingly advantageous fuel in the fuel cell for the miniaturized device. When the fuel cell using the liquid fuel is successfully miniaturized, a power source having a heretofore unattainable long service life can be realized for a miniaturized device.
Among other liquid fuels, such C1 to C2 compounds as methanol and ethanol are inexpensive and have moderately high boiling points and, therefore, can be readily used from the standpoint of safety. As a technical difficulty encountered by the fuel cell of this kind, the development of an electrode catalyst may be cited. In the anodic oxidation of such a fuel as methanol which contains carbon atoms, the phenomenon of poisoning due to the gradual fast deposition of the intermediate reaction product on the surface of electrode with the elapse of time manifests itself even when the electrode is made of platinum inherently exhibiting a high catalytic activity and eventually brings about a great loss of the catalytic activity of the electrode. This adverse phenomenon raises a serious problem in the development of a practicable fuel cell.
A campaign for the development of an electrode catalyst which excels in resistance to the poisoning in question is well under way. It has not yet succeeded in introducing a catalyst of ideal quality. At present, a fuel cell which is capable of stably yielding high output for a long time remains yet to be developed. The fuel cell which uses an organic fuel such as methanol, therefore, requires to curb the phenomenon of poisoning on the surface of the electrode.
As described above, the conventional fuel cell of ordinary run is complicated as a system and, unless the existing construction is given an appreciable improvement, poses a difficult problem in miniaturization. The conventional liquid fuel cell which makes use of the capillary action fits miniaturization from the constructional point of view. Since it is complicated and has many limitations in construction, it has not yet been miniaturized enough to be used as a power source for a small device. Further, the method for clamping the component parts of fuel cell, the discharge of the formed water, and the interconnection of electromotive parts need to be adapted for miniaturization of the fuel cell. The poisoning of the surface of electrode which occurs when a liquid fuel is used in the fuel cell also demands a due countermeasure.